In recent years, as awareness of global environment conservation increases, systems having a direct-current power supply such as a storage cell, a solar cell or a fuel cell are being developed. Those systems need a power supply device for converting direct-current (DC) power generated by a storage cell, a solar cell, a fuel cell, or the like to an alternating-current (AC) power and supplying the AC power to a load or a commercial power supply. In the case of charging a storage cell from a commercial power supply, it is necessary to convert the AC power of the commercial power supply to DC power and supply the DC power to the storage cell.
Japanese Unexamined Patent Application Publication No. 2002-78350 (Document 1) discloses a single-phase three-wire AC/DC bidirectional converter. The converter realizes charging of a cell from a single-phase three-wire AC power supply and power regeneration to the single-phase three-wire AC power supply by discharging the cell in a single circuit, aiming to realize smaller size and lower cost.
Japanese Unexamined Patent Application Publication No. 2008-289211 (Document 2) and Japanese Unexamined Patent Application Publication No. 9-140157 (Document 3) also disclose inverters adapted to a single-phase three-wire system.
In the case of performing backup of an AC load at the time of interruption of power supply of a commercial power system by using any of the power supply devices, which output a single-phase three-wire AC voltage, disclosed in Documents 1 to 3, the power supply device has to be in a standby state in which DC voltage is always supplied to the power supply device to promptly output the single-phase three-wire AC voltage from the power supply device.
However, two capacitors are connected in series between DC lines, which are DC input terminals, of each of those power supply devices and, generally, leakage current flows in the capacitors. In some cases, an auxiliary power supply for control is connected between the DC lines. Therefore, when the power supply device stands by in a state where DC voltage is supplied, DC power is consumed. In the case where the DC power supply is a battery, there is a problem that the battery gradually discharges.
There is a case that leakage current values vary among parts due to individual variability in two capacitors connected in series between DC lines. The voltages in the capacitors have to be balanced to some extent to maintain both of two capacitors connected in series in a charged state in order to promptly output a single-phase three-wire AC voltage. However, there is a problem that a loss will be large when a circuit for balancing voltages is provided and operated.
An object of the present invention is to provide a power supply device which has a small loss in a standby state when backup of an AC load is performed by outputting a single-phase three-wire AC voltage, and to provide a method of operating the device.